International Publication No. WO84/02882 discloses that among the compounds of formula (I), the compound wherein R is an alkyl having 2 to 5 carbon atoms, benzyl or phenethyl is useful as a developer for thermosensitive recording papers. Further, Japanese Patent Publication No. 61198/1988 discloses thermosensitive recording papers containing a compound of formula (I) wherein R is an alkyl having 6 to 20 carbon atoms.
These diphenyl sulfone developer compounds give improved coloring sensitivity, shelf life and light resistance in comparison with conventional phenol developers, and have recently been employed not only for recording papers for information, communication recording devices such as facsimile, etc. but also for thermosensitive bar cord labels for POS (Point of Sales) information management systems, various tickets, and the like.
As regards the methods of producing 4-substituted hydroxy-4'-hydroxydiphenyl sulfones which are monoether derivatives of 4,4'-dihydroxydiphenyl sulfone (hereinafter sometimes referred to as bisphenol S), methods which comprise reacting bisphenol S with a halide such as an alkyl halide or aralkyl halide in the presence of an alkali in a polar solvent such as dimethylformamide or alcohol are disclosed in Japanese Patent Unexamined Publication Nos. 20493/1983, 82788/1983, 13852/1985 and 56949/1985. These methods include the use of a polar solvent which dissolves the reactants and reaction products well, and are advantageous in that the charge weight per unit volume of reactor can be increased. However, the use of such solvent or dimethylsulfoxide gives rise to a difficulty in improving reaction selectivity, and also a diether derivative, i.e. 4,4'-di-substituted hydroxydiphenyl sulfone is formed in a non-negligible amount as a by-product. In addition, dimethylformamide, dimethylsulfoxide, etc. are relatively expensive so that recovery and recirculation steps need to be added.
Japanese Patent Unexamined Publication No. 255259/1988 proposes a method wherein a monoether derivative, i.e. 4-alkoxy-4'-hydroxydiphenyl sulfone, is produced by condensing an alkoxybenzenesulfonyl chloride and diphenyl carbonate in the presence of a Lewis acid, followed by hydrolysis of the carbonate bond. While this method solves the selectivity problem, it also poses various problems upon industrial application, such as lengthy processes, need to use phosgene which is highly toxic for the production of diphenyl carbonate, or the like.
Furthermore, Japanese Patent Unexamined Publication No. 225157/1982 discloses a method wherein a monobenzyl ether, i.e. 4-benzyloxy-4'-hydroxydiphenyl sulfone, is produced by dissolving 4,4'-dihydroxydiphenyl sulfone (1 mole) in water containing 0.8-1.2 equivalent of a hydroxide and/or a salt of alkali metal and/or alkaline earth metal, and reacting the same with 0.8-1.2 equivalent of benzyl chloride at 40.degree.-80.degree. C., followed by separation of resulting 4-hydroxy-4'-benzyloxydiphenyl sulfone. In this method, a monosodium salt is produced since the amount of the alkali relative to bisphenol S is 0.8-1.2 equivalent amount, but the monosodium salt has small solubility in water, necessitating increased amount of water which leads to low volume efficiency of the reactor and low reaction speed.
The object of the invention is to provide a method of producing 4-substituted hydroxy-4'-hydroxydiphenyl sulfones, having good selectivity and economical feasibility by simpler reaction steps, which shall overcome the problems as described.